FIELD OF THE INVENTION
The present invention relates to a method for synchronizing a received carrier to a transmitted carrier for a dispersed-energy QPSK signal.
The invention relates in general to the field of digital modulation methods, such as the digital video broadcast via satellite (DVB-S) method. In particular, the present invention relates to carrier recovery on reception of a transmitted digital signal, in particular of a dispersed-energy quadrature phase shift keying (QPSK) signal. A QPSK signal is the mixed product of two orthogonal signals I and Q (referred to as the I signal and the Q signal, respectively, in the following text), which are phase-shifted through 90.degree. with respect to one another. The I and Q signals are also completely independent of one another at the receiving end, provided the received carrier and the transmitted carrier are at the same frequency, and their phases are coupled.
At the receiving end, the QPSK signal is mixed in one path with a TI carrier signal in order to obtain the I signal, and is mixed in a further path with a TQ carrier signal in order to obtain the Q signal, which is phase-shifted through 90.degree. with respect to the carrier signal TI. The frequency of the carrier signal at the reception end must correspond precisely to the frequency of the carrier signal at the transmitter end, in order to ensure synchronous demodulation of the dispersed-energy QPSK signal. Non-matching carrier frequencies lead to rotation of the constellation containing the I signal and the Q signal. This rotation is brought to rest by suitable control of the received carrier, or, in modern methods, is compensated for by computational methods (for example the CORDIC algorithm). In this case, the method according to the invention can be used directly in analog QPSK demodulators by use of a comparatively simple circuit, with the minimum theoretically possible number of measured values or, using numerical QPSK processors, can determine the control value for the rotation compensation.
A known procedure for carrier recovery in the context of digital modulation methods is known, for example, from the reference titled "Digitale Modulationsverfahren" [Digital Modulation Methods], Rudolf Mausl, 1985/1991, ISBN 3-7785-2085-X. In chapter 3.4.2, the author describes the so-called COSTAS loop, under "carrier recovery". Based on the circuit shown in FIG. 3.12, the COSTAS loop for carrier recovery with two-phase keying contains the addition of a second multiplier. In the variant of the so-called "hard" COSTAS loop, the demodulation product of the in-phase and quadrature demodulator is supplied, after low-pass filtering, to one input of the two multipliers, and the demodulation product limited to the mathematical sign by a comparator is supplied, crossed over, to the other input. The difference between the two multiplication output signals is used to provide a correction voltage for controlling the voltage-controlled oscillator. The known circuit is balanced, that is to say it is based not only on SI-controlled detection of the complete SQ signal, but also on SQ-controlled detection of the complete SI signal.
The following reference should also be cited from the prior art: "Pulse Code Modulation Techniques", Bill Waggener, 1995, ISBN 0-442-01436-8. The chapter titled "Symbol Synchronization", pages 291 to 306, describes various complicated concepts for pulse code modulation, which are likewise balanced in the sense described above, and are based on a measurement of the overall signals.